In recent years, global warming due to the emission of greenhouse gases (CO2, etc.) associated with mass consumption of fossil fuels has posed a problem. In response, the realization of an energy supply system relying minimally on fossil fuels has been necessitated. A system for electric power supply by a hydrogen fuel cell, in particular, is an electric power supply system which does not discharge CO2 as an environmental load. Technologies for its coating are under study in many fields as an infrastructure for realizing hydrogen society aimed at sustained development. However, hydrogen as a fuel is a combustible gas involving explosion, and its handling requires a careful safety monitor. For this purpose, the development of an inexpensive detecting method, which safely detects leaking trace hydrogen, becomes one of the most important challenges in realizing hydrogen society. The trace of leaking hydrogen has hitherto been monitored using a hydrogen detector, or by confirming the amount of hydrogen consumption. However, prompt pinpointing of the location of leakage is difficult. Much labors and time are usually spent on the pinpointing. The general hydrogen gas detector measures a change in electrical resistance on the surface of a semiconductor due to the adsorption of hydrogen. Since an explosion is caused by ignition of hydrogen at the power circuit in general hydrogen gas detector, it has been problematic in terms of safety. Under these circumstances, proposals have been made for hydrogen leakage detecting methods (have been proposed) in which hydrogen leakage detecting paints containing fine particles (particle size: 1 μm or less) comprising palladium oxide or tungsten oxide, or hydrogen gas detecting materials coated with such paints are applied or stuck to a location suspected of hydrogen leakage (Patent Documents 1 to 3). Judgment of hydrogen leakage by the paint is based on the visual confirmation of the site of discoloration of palladium oxide or tungsten oxide by the adsorption of leaking hydrogen. Thus, this method is a hydrogen leakage detecting method which facilitates pinpointing of the location of leakage and has high safety.
However, the paint is applied as a film by a chemical manufacturing process using wide varieties of chemicals, including a strongly acidic aqueous solution, a strongly basic aqueous solution, and a harmful organic solvent. Thus, its manufacturing cost is high, and it is also problematical in terms of load on the surrounding environment and environmental sanitation for an operator. In recent years, moreover, there has been a concern about the effect on humans of inhalation of, or exposure to, fine particles with a particle size of 1 μm or less. The production and handling of such paints are not preferred for environmental sanitation for the operator.    Patent Document 1: JP-A-4-279681    Patent Document 2: JP-A-8-253742    Patent Document 3: JP-A-2005-345338